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SO tolerant Pt/TiO catalysts for CO oxidation and the effect of TiO supports on catalytic activity 2

Kenji Taira, Kenji Nakao, Kimihito Suzuki, and Hisahiro Einaga Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01652 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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Environmental Science & Technology

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SOx tolerant Pt/TiO2 catalysts for CO oxidation and the effect

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of TiO2 supports on catalytic activity

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Kenji Taira+§*, Kenji Nakao+, Kimihito Suzuki+, Hisahiro Einaga§ +

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Advanced Technology Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba, 293-8511 Japan

§

Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen Kasuga-city Fukuoka, Japan *

Corresponding Author

E-mail: [email protected] Tel.: +81 70 3914 4689. Fax: +81 439 80 2745

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ABSTRACT: We developed a new technique for mitigating catalyst deactivation caused by SO2 in ex-

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haust gases. A series of 0.1 wt%-Pt/TiO2 catalysts with different surface, crystal and pore structures

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were prepared and tested for CO oxidation activity in the presence of SO2 and H2O. The order of the CO

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oxidation activity under the influence of SO2 was much different from that in the absence of SO2. Cata-

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lysts with a high ratio of larger pores exhibited higher catalytic activity under the influence of SO2 and

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H2O in the temperature range of 250-300oC, whereas other parameters, such as BET surface area and

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crystal structure of the TiO2 support, had minor effects on the CO oxidation activity. The oxidation

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state of Pt differed significantly depending on the kind of TiO2 support. Some catalysts were less active

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without H2 reduction pretreatment due to the presence of oxidized Pt species.

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KEYWORDS. CO oxidation, Pt, TiO2, catalyst, SO2

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ACS Paragon Plus Environment


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Introduction

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We have long relied on fossil fuels as an energy source. Despite continuous improvements in the

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fuel combustion processes, some of the fuel still remains in the exhaust gases after combustion, and the-

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se fuels cause environmental issues. Exhaust gas from the sintering process of the steel industry includes

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about 1vol% of CO. It is preferable to remove CO from the exhaust gas with end-of-pipe catalytic com-

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bustion. However, toxic SOx is an unavoidable component of the exhaust gas from coal combustion be-

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cause coal contains sulfur as an impurity. Catalysts are severely deactivated by SO2, regardless of their

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composition 1,2. For example, non-noble metal catalysts, such as CeO2-based catalysts, lose their activity

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3

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the sulfate Ce(SO4)2, which alters the surface properties of the catalyst.

or selectivity 4 under the influence of SO2. In this reaction, SO2 oxidizes on the oxide surface to form

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Much research has been conducted into the effects of SO2 on the activity of noble-metal cata5-16

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lysts

. Pd/Al2O3, a widely-used combustion catalyst, has been reported to suffer from severe deacti-

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vation because Pd reacts with SO2 to form PdSO4 5,6. While the catalytic activity of Pd/Al2O3 is partially

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recovered by the elimination of SO3 from the Pd surface to the Al2O3 during the reaction, this recovery

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process is inhibited by water vapor in the reaction gas 5,6. Thus, the co-existence of SO2 and H2O pro-

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motes the deactivation of Pd/Al2O3.

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The tolerance of noble metals against SO2 strongly depends on the kinds of metals. Pt-based

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catalysts show higher catalytic activity for CO oxidation than Pd-based catalysts in the presence of SO2

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7

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based catalysts were deactivated in the presence of SO2

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SO3, which migrated to the surface of the supports, giving rise to surface sulfates

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sulfate formed on the supports depends on the reaction temperature, and it significantly increases as the

because the SO3 species formed on Pt are more easily removed than that on Pd 8. However, even Pt 9,10

, because SO2 was oxidized on Pt to form

2


10-13

. The amount of

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reaction temperature decreases. Typically, the amount peaks at a reaction temperature around 250oC

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and the removal of the sulfur species from the supports requires thermal treatment at temperatures above

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350oC 13. Some studies have found that sulfates on acidic oxides, i.e. SiO2, TiO2 or ZrO2, are less stable

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and can be removed from the supports more easily than Al2O3 14,15. TiO2 and ZrO2 also have advantages

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over the other supports in that the oxidation of SO2 to SO3 can be inhibited: Pt catalysts supported on

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TiO2 and ZrO2 are less susceptible to SO2 oxidation than those on SiO2 or Al2O3 16. On the basis of the-

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se findings, TiO2-supported Pt catalysts are good candidates for the CO oxidation catalysts in the pres-

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ence of SO2. However, there have been no reports on which physical or chemical properties of the sup-

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ports strongly affect the SOx tolerance of Pt/TiO2 catalysts. Furthermore, only a few papers have been

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published on the results of oxidation reactions under a mixture of SO2 and H2O at a reaction temperature

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of around 250oC, which is a harsh reaction condition where the catalysts are severely poisoned by SO2.

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In this report, we prepared Pt/TiO2 catalysts with a series of TiO2 supports having various surface areas

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and pore structures and then compared their catalytic activity. The reaction gas was simulated exhaust

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gas from the sintering process in the steel industry, which includes 40 ppm of SO2 and 20vol% of H2O.

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Experimental Section

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Catalyst preparation

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All of the Pt/TiO2 catalysts were prepared by the impregnation method using H2PtCl6 as the

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metal source. The TiO2 supports were supplied by Catalysis Society of Japan (CSJ) or a commercial

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source. The supports used were JRC-TIO-2 (CSJ, BET area: 17 m2/g, pore volume (PV): 0.12 cm3/g),

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JRC-TIO-4 (CSJ, BET area: 47 m2/g, PV: 0.33 cm3/g), JRC-TIO-6 (CSJ, BET area: 107 m2/g, PV: 0.54

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cm3/g), JRC-TIO-7 (CSJ, BET area: 270 m2/g, PV: 0.38 cm3/g), ST-01 (Ishihara Sangyo, BET area: 285 3



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m2/g, PV: 0.42 cm3/g), FTL-110 (Ishihara Sangyo, BET area: 15.8 m2/g, PV: 0.06 cm3/g) and FTL-200

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(Ishihara Sangyo, BET area: 7.6 m2/g, PV: 0.03 cm3/g). The supports were dried at 110oC for 10 h and

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then heated at 500oC in air for 1 h before use. The BET surface areas of the TiO2 decreased after these

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treatments.

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All samples were prepared by incipient wetness method with careful operation (see S2 for de-

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tails). Precursor solutions of H2PtCl6 6H2O (Sigma Aldrich, > 99.995%) were added dropwise to 1.00 g

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of each TiO2 support and mixed thoroughly at room temperature. Then, the samples were dried at 110oC

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overnight and calcined at 500oC for 1 hour. The catalysts were denoted as follows corresponding to their

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supports: Pt/TIO-2, Pt/TIO-4, Pt/TIO-6, Pt/TIO-7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200. For example,

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Pt/TIO-2 was prepared using JRC-TIO-2. The amount of Pt was calculated to be 0.1wt% as metal Pt

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based on the amount of the precursor used. For the X-ray photoelectron spectroscopic (XPS) experi-

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ments, catalysts with 1wt% of Pt were also prepared with three supports: TIO-2, TIO-4 and TIO-6. All

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of the catalysts were prepared following the same procedure as the catalysts with less Pt except that

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concentrated H2PtCl6 solution was used. The prepared catalysts are hereinafter referred to as Pt1/TIO-2,

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Pt1/TIO-4 and Pt1/TIO-6.

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Catalyst characterization

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The adsorption isotherm of N2 was measured at the temperature of liquid nitrogen (-196ºC) with

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an adsorption measurement instrument (Japan BEL, BEL-max). The surface area of the catalysts was

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determined by the Brunauer-Emmett-Teller (BET) method from the N2 adsorption isotherm. The pore

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volume of the catalysts was estimated by the amount of N2 adsorbed on the catalysts at a relative pres-

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sure of p/po = 0.990. The pore distribution was calculated by the Dollimore-Heal (DH) method using the

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isotherm data 17.

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X-ray diffraction (XRD) measurements were performed on all of the prepared catalysts using an

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XRD instrument (Rigaku, RINT-TTR III). Scanning proceeded from 2θ = 20o to 60o with a 0.02o step

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angle at a scanning rate of 1o/min. The X-ray tube voltage was 40 kV and the current was 150 mA. The

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ratios between the rutile phase and anatase phase of the TiO2 in the catalysts were estimated using a

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previously reported procedure18.

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Dispersion of Pt was estimated by the CO pulse chemisorption method with a catalyst analyzer

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(Japan BELL, BEL-CAT II). The pretreatment procedure was basically determined with reference to

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other reports 19,20 and the dispersion was calculated under the assumption that CO:Pt = 1:1 21. However,

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the calculated Pt dispersion values were unacceptably small when the CO pulse chemisorption was per-

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formed directly after the H2 reduction step (Table S1) due to the strong-metal-support-interaction

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(SMSI) like behavior

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chemisorption to recover the Pt sites from the SMSI like state to the normal states 24. A detailed expla-

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22,23

. Therefore, we added a re-oxidation step in diluted air flow before CO pulse

nation of the measurement procedures is included in the caption for Table S1.

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TEM images of the Pt/TiO2 catalysts were taken with a transmission electron microscope (FEI,

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Tecnai G2) to further assess the Pt dispersion. All of the images were measured as bright field images.

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The accelerating voltage was adjusted to 150 kV, and the average Pt particle sizes were determined us-

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ing the procedure described in the caption for Fig. S1.

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Fourier Transform Infrared (FTIR) spectra were obtained using a Nicolet iS50 FT-IR (Thermo

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scientific) in Attenuated Total Reflection (ATR) configuration with a diamond prism. The spectra were

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recorded with a 2 cm-1 step and 64 times of the cumulated number. The differential spectra were calcu-

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lated between the catalysts before and after the CO oxidation reaction test.

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Thermal gravimetric analysis (TGA) profiles were recorded from room temperature to 850oC

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with a thermal analysis instrument (Shimadzu, TG-50) for the catalysts before and after the CO oxida-

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tion reaction tests. All of the measurements were performed in a N2 flow of 50 cm3/min.

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The oxidation states of Pt in Pt1/TIO-2, Pt1/TIO-4 and Pt1/TIO-6 were estimated by X-ray pho-

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toelectron spectroscopy (XPS). All of the spectra were taken in a XPS analyzer (ULVAC-Phi, Quan-

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tum-2000) equipped with a monochromated Al X-ray source and a charge neutralizer. All of the meas-

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urements were performed at a pass energy of 29.35 eV and recording step of 0.125 eV. The peak shift

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derived from charge up of the catalysts was corrected by adjusting the binding energy of the C1s peak to

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285.0 eV. Spectra of Ti2p and O1s were also recorded to assess the validity of the adjustment. The de-

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viation of the peak-top positions between the catalysts was confirmed to be below 0.2 eV.

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Catalytic CO oxidation studies

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The catalytic activity of the Pt/TiO2 catalysts was evaluated by the CO oxidation reaction (CO +

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1/2O2 → CO2) at atmospheric pressure. A schematic of the reactor is shown in Fig. S2. The gas compo-

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sition was controlled with mass flow controllers, and water was introduced with a water pump (Nihon

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Seimitsu Kagaku, NP-KX-510). The water pump nozzle was kept at 115oC, and all of the water was va-

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porized on the upstream side of the reaction tube. The catalysts were packed into a quartz glass tube,

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and their temperatures were monitored with a K-type thermocouple. In order to increase the thickness of

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the catalyst layer, 10 mg of Pt/TiO2 catalyst was diluted with 20 mg of the same TiO2 support. This dilu-

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tion operation was confirmed not to affect the catalytic activity but to ease the sample packing. A reduc6


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tion pretreatment was performed at 500oC for 30 minutes in H2 before each reaction test. Some of the

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catalysts were used without a reduction pretreatment. The gas composition was CO: 1%, O2: 10%, H2O:

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20%, NO: 40 ppm and SO2: 40 ppm, with N2 making up the balance. The total amount of gas flow was

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adjusted to 100 cm3/min, of which the Space Velocity (SV) was 600,000 cm3/h.gcat. Typically, the reac-

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tions were performed at 250oC for 6 to 20 h. The gas composition of the outlet gas from the reactor was

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determined with a GC-TCD (Shimadzu, GC-2014) or infrared gas analyzer (Yokogawa Electric, IR-

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200) every 25 minutes, and the CO conversion was calculated using the CO and CO2 concentrations.

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We confirmed that the values measured by both devices were consistent (Table S2). To determine the

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dependence of the reaction rate on reaction temperature, we performed reaction tests with higher SV

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(6,000,000 or 60,000,000 cm3/h.gcat). The other details of the experimental conditions are described in

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the caption for Fig. S3.

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Results and discussion

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Effect of crystal structure and Pt dispersion on catalytic activity

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Table 1 summarizes the textural properties of the Pt/TiO2 catalysts used in this study. TEM im-

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ages showed that Pt particles with a size of 1.1-1.6 nm were dispersed on the TiO2 particles (Figure S1).

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The Pt dispersion values estimated from the CO chemisorption were similar for each of the Pt/TiO2

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catalysts and in the range of 42-55%. According to the Pt dispersion, the average Pt particle sizes were

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estimated to be 2.1-2.7 nm, which were consistent with the sizes determined from the TEM observation.

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It is worth noting that none of the Pt dispersion determined by the techniques correlates with the catalyst

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surface area or the ratio of anatase/rutile in the TiO2 support. 7



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Fig. 1 shows the time course profiles for CO oxidation of the Pt/TiO2 catalysts at 250oC in the

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presence of SO2 and H2O. Although the initial conversions were comparable for all of the catalysts, the

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stability varied depending on the catalyst support. Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6 showed a stable CO

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conversion around 60 % for as long as 19 hours, while the values of the other catalysts dropped to

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around 10%. Thus, the catalytic activities of Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6 for CO oxidation were

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much higher than those of Pt/TIO-7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200 in the presence of SO2. The

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total amount of SO2 fed to the reactor was 9.8 × 10-6 mol after 1 h, which corresponds to 0.96 mg or 5.2

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× 10-4 cm3 of H2SO4 assuming complete conversion of SO2 to H2SO4. This amount of H2SO4 is compa-

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rable to a pore volume of 6 × 10-4 cm3, which is equivalent to 10-mg Pt/FTL-200 used in the activity test.

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The total turnover number for CO conversion was 104 orders larger than that of the exposed Pt amounts

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(Table S3), indicating that the CO oxidation catalytically proceeded for all the Pt/TiO2 catalysts.

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As described above, the Pt/TiO2 catalysts were deactivated during CO oxidation in the presence

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of SO2 and H2O. When the reaction was carried out in the absence of SO2 at 250°C, all of the catalysts

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exhibited steady CO oxidation without any decrease in the catalytic activity. Under these conditions, the

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CO oxidation activity decreased in the order of Pt/TIO-6 > Pt/TIO-7 > Pt/FTL-200 > Pt/ST-01 >

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Pt/TIO-4 > Pt/TIO-2 > Pt/FTL-110 (Table S4). However, the order of the catalytic activity of Pt/TiO2

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in the presence of SO2 and H2O varied significantly from that in the absence of SO2. Therefore, the fac-

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tors controlling the catalytic activities of Pt/TiO2 in the presence of SO2 and H2O differed from those in

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the absence of SO2.

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To clarify the factors affecting the catalytic activity of Pt/TiO2 in the presence of SO2 and H2O,

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we investigated the relationship between the catalytic activity and the textural catalyst properties listed

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in Table 1. Assuming the adherence of SO2 on the Pt surface is a major cause of catalytic deactivation,

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we first speculated that higher Pt dispersion may lead to higher tolerance against SO2. In the present 8


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study, however, the difference in Pt dispersion among the catalysts was small and not reflected in the

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catalytic activity for CO oxidation in the presence of SO2. Moreover, Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6,

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which exhibited higher catalytic activity, did not have larger Pt dispersion values than the other cata-

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lysts. The TiO2 supports used in this study had various anatase/rutile ratios in a range of 1/0 to 0/1 (Ta-

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ble 1, Fig. S4). Both the catalysts with higher activities (Pt/TIO-2, Pt/TIO-4, Pt/TIO-6) and those with

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lower activities (the rest of the catalyst group) had anatase and rutile phases. Therefore, the ana-

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tase/rutile ratio in the TiO2 support was not an important factor for controlling the catalytic CO oxida-

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tion activities of Pt/TiO2 in the presence of SO2 and H2O.

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SO42- species after catalytic CO oxidation

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SO42- species formed on the catalyst surface during CO oxidation in the presence of SO2 because

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the reaction gas included not only SO2, but also O2 and H2O. The following side reactions proceeded

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along with the CO oxidation reaction. Pt/TiO2 catalysts can catalyze the oxidation of SO2 to SO3 (eq(1)),

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followed by the hydration of SO3 (eq(2)).

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SO2 + 1/2 O2 → SO3

(1)

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SO3 + H2O → H2SO4

(2)

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The boiling point of sulfuric acid strongly depends on its concentration and can reach over 300oC at

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around 98% 25,26. This indicates that by-product sulfuric acid could remain as a liquid on the surface of

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the catalysts during CO oxidation at 250ºC.

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Subsequently, we focused on the chemical state of S-containing species that formed on the cata-

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lyst surface during CO oxidation. Figure S5 shows the differences in the FTIR spectra of the Pt/TiO2 9



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catalysts, which were obtained by subtracting the spectra of Pt/TiO2 before reaction from those after re-

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action. The bands were formed in the wavenumber range of 850-1300 cm-1 with the peaks at 1230,

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1149, 1043, 982 cm-1, which are the characteristic bands of bidentate SO42- species on Ti4+ sites.27 This

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indicates that SO2 was oxidized on the Pt/TiO2 catalysts to form SO42- species on the catalyst surface,

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and this is considered to be the cause of catalyst deactivation.

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The amount of the SO42- species on the catalysts after the reaction was estimated from the ther-

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mogravimetric studies. Figure S6 shows the TGA profiles of the Pt/TiO2 catalysts that were used for

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CO oxidation at 250ºC for 6 h. The catalyst weight decreased with increasing catalyst temperature in

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the range of 20-400ºC due to desorption of water from the catalyst. The weight loss in the temperature

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range of 400-800ºC was ascribed to the desorption of SO42- species from the catalyst surface

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amount of SO42- species was estimated from the weight changes in this temperature range and listed in

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Table S5. The amount of SO42- that formed on the Pt/TiO2 catalyst was calculated to be roughly propor-

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tional to its BET surface area. In addition, Pt/ST-01, a catalyst with lower activity, had a lower amount

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of SOx than the higher-activity catalysts, Pt/TIO-2 and Pt/TIO-4. The catalytic properties of Pt/TiO2 in

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the presence of SO2 and H2O cannot be explained in terms of the amount of SO42- adsorbed on the cata-

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lysts.

28

. The

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Effect of pore distribution on catalytic activity

213 214

The vapor pressure of sulfuric acid on the porous catalyst surface obeys the Kelvin equation (3)

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29

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contact angle, rp is the pore radii, R is the gas constant, and T is the temperature.

, where p/po is the relative pressure, VL is the molar liquid volume, γ is the surface tension, θ is the

10


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௣

ln ቀ ቁ = − ௣೚

ଶ௏ಽ ఊୡ୭ୱఏ

(3)

௥೛ ோ்

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The Kelvin equation states that the condensation of sulfuric acid occurs in smaller pores first and that

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larger pores are less vulnerable to condensation. In fact, one study on activated carbon particles found

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that sulfuric acid filled nanopores first before spilling over to the larger pores 30. Based on these results,

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we postulated that pore blockage occurs due to by-product sulfuric acid on the Pt/TiO2 catalyst. A

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schematic of this hypothesis is shown in Fig. 2. In our experiment, the formation of sulfuric acid was

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accelerated on the Pt sites, and the nanopores and mesopores were filled or blocked by the sulfuric acid.

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TEM and SEM images showed that the Pt/TiO2 catalysts were composed of agglomerated TiO2 particles,

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and pores were formed as the spaces between the particles (Fig. S1, Fig. S7). Therefore, pore blockage

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by sulfuric acid can retard the diffusion of reaction gases in the secondary particles for all of the Pt/TiO2

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catalysts, and the reaction gases cannot access the Pt sites inside the pores, leading to the catalyst deac-

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tivation.

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According to the pore-blockage mechanism, catalysts with larger pores should be able to exhibit

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stable catalytic activity in the presence of SO2. To verify this hypothesis, we calculated the pore distri-

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bution of each catalyst using the Dollimore-Heal (DH) method 17. The pore size distribution is shown in

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Figs. 3(a) and (b). Peaks appeared at rp > 10 nm for Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6, whereas Pt/TIO-

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7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200 had peaks at rp < 10 nm. These results indicate that the cata-

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lysts with larger pore sizes exhibited higher CO oxidation activity than those having smaller pore sizes,

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suggesting that pore-blockage by the sulfuric acid is a key factor controlling the catalytic properties of

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Pt/TiO2.

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The above results imply that Pt particles located in pores smaller than 10 nm cannot take part in

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the oxidation reaction because CO and O2 cannot be adsorbed on the Pt sites. To further discuss the ef11



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fect of the TiO2 support pore sizes, we defined a parameter to quantitatively evaluate the pore blockage

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level. Based on the TEM images, the Pt particles were shown to be highly dispersed on the surface of

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supports (Fig. S1). Therefore, it is possible to evaluate the tolerance against SO2 using the relative sur-

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face area, specifically, the ratio of surface area attributable to small pores against the total surface area.

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Pore distribution was recalculated in terms of surface area, and the cumulative sum of the surface area,

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A0-rp, was calculated from the smaller rp. Every cumulative sum was normalized by the cumulative total

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value of rp < 100 nm, A0-100, of each catalyst. Figure 4 shows the results of relative surface area calcula-

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tions, A0-rp/A0-100, for all Pt/TiO2 catalysts. The catalysts can be divided into two categories in terms of

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A0-rp/A0-100: Pt/FTL-110, Pt/FTL-200, Pt/ST-01 and Pt/TIO-7, which are in the lower activity group and

248

have a larger A0-rp/A0-100, and Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6, which belong to the higher activity

249

group and have a smaller A0-rp/A0-100. For example, Pt/TiO2 catalysts can be arranged in descending or-

250

der of A0-rp/A0-100 at rp = 10 nm as follows: Pt/FTL-110～Pt/TIO-7 > Pt/ST-01 > Pt/FTL-200 > Pt/TIO-4

251

> Pt/TIO-2 > Pt/TIO-6. This order is in agreement with the results of the catalytic activity test shown in

252

Fig. 1. The catalysts with larger A0-rp/A0-100 values exhibited deceased CO oxidation reaction activity.

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Another piece of evidence for the pore blocking mechanism was obtained from the effect of the

254

reaction temperature on the catalytic activity of Pt/TiO2. The saturated vapor pressure of sulfuric acid,

255

po in equation (3), increases as the reaction temperature rises

256

tion is caused by pore blockage with sulfuric acid, it seems reasonable to expect that temperature in-

257

creases will lift the blockage at around the boiling point of sulfuric acid, which is estimated to be >

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300oC, and lead to higher catalytic activity. In addition, the leap in catalytic activity due to the rise in

259

temperature is expected to be more evident for catalysts with small pores than for those with large pores.

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Figure 5 shows the temperature dependence of catalytic activity for all catalysts. The catalysts Pt/TIO-2,

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Pt/TIO-4 and Pt/TIO-6 exhibited higher catalytic activity as shown in Fig. 1, while the other catalysts

25 26

, . Under the assumption that deactiva-

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Pt/TIO-7, Pt/ST-01, Pt/FTL-110 and Pt/FTL-200 showed lower catalytic activity. Here, the SV of the

263

reaction is 10 times larger than that in Fig. 1. The CO conversion values of the catalysts in the former

264

group are larger than those of the latter group at temperatures lower than 300oC. The CO conversion of

265

the former group rose smoothly over the entire range of the experiment. However, the CO conversion of

266

the latter group soared at temperatures around 300oC to 315oC and reached a similar CO conversion to

267

that of the former group.

268

To further investigate the effect of SO2 on the CO oxidation activity of Pt/TiO2 catalysts, we car-

269

ried out kinetic studies of CO oxidation with Pt/ST-01, whose activity was greatly affected by SO2. Fig-

270

ure S3 and Table S6 show the Arrhenius plots for CO oxidation at lower temperatures of 250-270oC and

271

higher temperatures of 325-350oC in the presence of SO2 and H2O. In both cases, a linear relationship

272

was observed, and the apparent kinetic energy was calculated to be 151 kJ mol-1 for the higher tempera-

273

tures and 143 kJ mol-1 for the lower temperatures. The comparable values for both cases show that the

274

rate determining steps are identical for both temperature ranges but that the number of active sites are

275

different as a result of pore blockage by H2SO4 at the lower temperatures of 250-270oC. .

276

To add another confirmation for the validity of the pore blockage hypothesis, we carried out CO

277

oxidation using Pt/ZrO2 catalysts with various pore sizes. ZrO2 is also tolerant against SOx and H2SO4.

278

Therefore, H2SO4 can remain liquid and deactivate the Pt/ZrO2 catalysts by pore blocking. As a result,

279

the same tendency was observed for Pt/ZrO2 catalysts as the Pt/TiO2 catalysts. The Pt/ZrO2 catalyst

280

with pores larger than 10 nm, Pt/ZRO-4, exhibited much higher CO oxidation activity than the catalysts

281

with pores smaller than 10 nm, Pt/ZRO-3, 5, 6 (Fig. S8-10). The catalytic activity of the Pt/ZrO2 cata-

282

lysts also showed a similar dependence on the reaction temperature as the Pt/TiO2 catalysts (Fig. S11).

283

In addition, CO conversion activity of catalysts with a similar pore distribution showed an almost iden-

284

tical dependence on the reaction temperature regardless of whether TiO2 or ZrO2 supports were used. 13



Page 14 of 25

285

Pt/TIO-2 and Pt/ZRO-4, which have peaks at around rp = 20 nm, exhibited a gradual increase in CO

286

conversion activity, while Pt/TIO-7 and Pt/ZRO-3, which have peaks at around rp = 6 nm, soared at

287

315-320oC (Fig. 5, Fig. S11). These results indicate that the pore-blockage hypothesis is correct and cat-

288

alysts with larger pores are more robust against SO2. Also, the theory has generality across a variety of

289

supports.

290 291

Oxidation state of Pt and its effect on catalytic activity without pre-reduction treatment

292

As discussed in the previous section, the catalysts Pt/TIO-2, Pt/TIO-4 and Pt/TIO-6 exhibited

293

higher catalytic activity than the other catalysts in the presence of SO2 and H2O. We subsequently inves-

294

tigated the effect of the catalyst pretreatment on the catalytic properties of Pt/TiO2 to shed light on the

295

impact of the oxidation state of the Pt. Figure 6 shows the time course for CO oxidation by Pt/TIO-2,

296

Pt/TIO-4 and Pt/TIO-6 without H2 reduction pretreatment. Pt/TIO-2 exhibited a similar CO conversion

297

rate regardless of whether the reduction treatment was performed. On the other hand, Pt/TIO-4 and

298

Pt/TIO-6 exhibited much lower CO oxidation activity compared with the reaction rate after H2 reduc-

299

tion.

300

We performed XPS measurements to study the oxidation states of the Pt particles before reduc31

301

tion treatment

. To obtain a sufficient peak intensity of Pt4f, we prepared Pt1/TIO-2, Pt1/TIO-4 and

302

Pt1/TIO-6 with a Pt loading level of 1.0wt%. The XPS measurement results are plotted in Fig. S12. The

303

Pt dispersion of Pt1/TIO-2, Pt1/TIO-4 and Pt1/TIO-6 were 46.9%, 44.9% and 53.4%, respectively, ac-

304

cording to the CO pulse chemisorption measurements. Furthermore, the Pt-particle sizes of Pt1/TIO-2,

305

Pt1/TIO-4 and Pt1/TIO-6 were 1.4 nm, 1.6 nm and 1.7 nm, respectively, based on the results of the

306

TEM measurements. These values are almost identical to those of the lower-Pt counterparts in Table 1. 14


Page 15 of 25


307

Additionally, to evaluate the effect of Pt loading per unit surface area on the oxidative state of Pt, an

308

XPS measurement was also performed on Pt/TIO-2, which has a lower surface area than the rest of the

309

catalysts. A Pt4f spectrum of Pt/TIO-2 was obtained as a difference spectrum between the spectrum of

310

Pt/TIO-2 and that of TIO-2 (Fig. S13). The difference spectrum of Pt/TIO-2 had an almost identical

311

spectrum to that of Pt1TIO-2. Therefore, it is reasonable to estimate the oxidative state of Pt on the cata-

312

lysts with 0.1wt% of Pt using catalysts with 1.0wt% of Pt. In the spectrum for Pt1/TIO-2 in Fig. S12,

313

the peak for Pt4f7/2 appeared around the binding energy of 71.9 eV and a peak corresponding to Pt4f5/2

314

was observed around 75 eV. No other peaks were detected, even after peak separation of the spectrum.

315

In contrast, Pt1/TIO-4 and Pt1/TIO-6 had clear peaks around the binding energy of 78 eV, which is as-

316

signed to Pt4f5/2 of oxidized Pt 32. A peak corresponding to Pt4f/7/2 was detected over the binding energy

317

of 74 eV. Moreover, the Pt1/TIO-6 spectrum had no Pt4f7/2 peak around 72 eV, but it did have a peak

318

around 73 eV. These results indicate that the better part of Pt in Pt/TIO-6 is more highly oxidized than

319

the other catalysts. Oxidized Pt has lower catalytic activity than metallic Pt31. The existence of oxidized

320

Pt is assumed to be the prime cause of the lower catalytic activity of Pt/TIO-4 and Pt/TIO-6 without re-

321

duction pretreatment.

322

The above results show that Pt/TIO-2 is a good candidate for a CO oxidation catalyst in the

323

presence of SO2 because the catalyst exhibits high activity without H2 treatment. For the Pt/TIO-2 cata-

324

lyst, the Pt species is more resistant against oxidation compared with those on the other catalysts, and Pt

325

was present in the reduced form even after heat treatment in air. On the other hand, the other catalysts

326

needed H2 treatment to achieve high performance. One possible explanation for the difference between

327

these catalysts is ascribed to the difference in the crystal structures of TiO2. Pt/TIO-2 catalysts have ana-

328

tase phases, whereas Pt/TIO-6 catalysts have the rutile phase and Pt/TIO-4 catalysts have the both phas-

329

es33. Oxygen supplied through oxygen vacancies plays an important role in oxidizing noble metals on 15



Page 16 of 25

330

the oxide supports 34. The number of TiO2 oxygen vacancies is reported to descend in order of: TIO-6,

331

TIO-4, TIO-2

332

phase

333

structure. Moreover, the unit cell has an almost identical size to that of rutile TiO2 (Table S7) 37,38. The

334

mismatch of a and b axes between rutile TiO2 and PtO2 remains 2.4%, which is small enough to ensure

335

lattice matching. Thus, oxygen supply from the vacancies and the epitaxy effects from the support are

336

considered to play important roles in oxidizing the Pt on rutile TiO2.

36

35

. Moreover, the rutile phase is more prone to create oxygen vacancies than the anatase

. Another possible cause of Pt oxidation is the epitaxy effect. PtO2 has a rutile or quasi rutile

337

We also considered the preparation method of the TiO2 supports. TIO-2 and TIO-6 were pre-

338

pared by the sulfuric acid decomposition method, and they include sulfur as an impurity (Table S8). Sul-

339

fur clearly remains in TIO-2 during the catalyst preparation process because TIO-2 was calcined at

340

900oC in the preparation process 39. On the other hand, TIO-4 was prepared by the chlorine method, and

341

it contained no detectable sulfur (Table S8)39. In addition, TiO2 supports prepared by the chlorine meth-

342

od are reported to have a tendency to oxidize the Pt deposited on them

343

prone to oxidization on TIO-4 than TIO-2 due to the difference in the preparation method as well as the

344

crystal structure.

40

. Pt is considered to be more

345

In this study, we investigated the relationship between the textural and catalytic properties of a

346

series of Pt/TiO2 catalysts for CO oxidation at relatively low temperature (250ºC) in the presence of SO2

347

and H2O. On the basis of the results described above, we concluded that the pore structure of Pt/TiO2

348

catalysts is one of the important factors controlling the activity because the adsorption-desorption of

349

SO2 is the key step for the reaction. The catalysts with larger pores than 10 nm are suitable for the reac-

350

tion under the influence of SO2 when the reaction was carried out at the temperatures than 500ºC.

351 16


Page 17 of 25


352 353

Supporting Information Experimental details of the sample preparation method, Dependence of CO

354

pulse chemisorption results on the pretreatment method, TEM images of Pt/TiO2, schematics of the re-

355

actor, CO conversion calculated by GC-2014 (Shimadzu) and IR-200 (Yokogawa Electric), amount of

356

CO2 produced in the reaction test, CO conversion at 250oC in absence of SO2, Arrhenius plot of CO ox-

357

idation reaction, XRD spectra of Pt/TiO2, FT-IR spectra of catalysts, TGA results, SEM images of

358

Pt/TiO2 catalysts, data lists used for Arrhenius plot, experimental results relating to Pt/ZrO2 catalysts, ,

359

XPS Pt4f spectra of catalysts, structural data of PtO2, chemical composition of TiO2 supports.

360 361

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1 2

Page 20 of 25

Table 1. Physical properties and Pt dispersion and the particle diameter determined by CO pulse chemisorptions of prepared catalysts TEM Catalyst

Support

BET area (m2/g)

Pore Volume

CO pulse

Pt particle Pt dispersion Pt particle size (nm) (%) size (nm)

(cm3/g)

Pt/TIO-2

anatase

15.6

0.17

1.4

46.7

2.4

Pt/TIO-4

anatase 88%

50.2

0.55

1.5

46.6

2.4

rutile 12% Pt/TIO-6

rutile

44.1

0.44

1.5

52.5

2.2

Pt/TIO-7

anatase

82.2

0.39

1.4

43.8

2.6

Pt/ST-01

anatase

88.1

0.37

1.1

42.4

2.7

Pt/FTL-110

anatase 2%

13.6

0.08

1.6

48.7

2.3

9.6

0.06

1.6

54.3

2.1

rutile 98% Pt/FTL-200

anatase 2% rutile 98%

3 4 5


Page 21 of 25


6 7 8 9 10

Figure 1. Time dependence of CO conversion Reaction temperature: 250oC, Pressure 1 atm, Gas composition: 1% CO, 10% O2, 20% H2O, 40 ppm SO2, 40 ppm NO and N2 balance, GHSV: 600,000 cm3STP g-1cat h-1

11

12 13

Figure 2. Schematics of pore blockage hypothesis

14



Page 22 of 25

15 16

Figure 3. Pore distribution of each Pt/TiO2 catalyst determined by DH method

17 18

Results of Pt/TIO-6, Pt/TIO-7 and Pt/ST-01 are shown in (a), and those of Pt/TIO-2, Pt/TIO-4, Pt/FTL110 and Pt/FTL-200 are shown in (b).

19


Page 23 of 25


20 21 22

Figure 4. Ratio between surface areas of pores smaller than each rp (A0-rp) against the surface area attributed to the pores of rp

TiO2 Catalysts for CO Oxidation ... - ACS Publications

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